Is it being watered adequately?

Yes. My thoughts is it might be aphid or a fungus. I was hoping to hear from other spirea owners.

https://gardening.yardener.com/Problems-Of-Spirea

Can you show the whole shrub from a little further back so we can see its surroundings? i think you can discount aphids. They don't produce that effect. It looks more fungal which could be water related. While we’re deliberating it would profit from a trim to include deadheading.

I’m hoping this is what you are looking for.

This one might be better

it doesnt matter if anything is attacking the leaves now .. its what caused such severe stress that the leaves were damaged.... allowing for some secondary thing to come along ...

we must define the primary issue.. not the secondary issue ...

it looks like severe lack of water ... take a hand trowel ... and dig 3 or 4 holes close around the plant.. down 6 inches or so.. and find out about moisture at root level ... and then water accordingly ...

did you bare root the plants at planting.. did you do anything about winding roots.. if any ...

it really doensnt matter that its twin.. a few feet away is doing fine.. other than that one is probably getting enough water to its roots ...

after perfecting water ... thats what its going to look like for the rest of the season.. then it will lose its ugly leaves... but all should be well in spring ... but for whatever ma nature throws at it during winter.. but we sure dont want it going into winter dry ...

ken